1. Field of the Invention
The present invention relates to a boost pressure control system for an engine having a supercharger and an air bypass valve.
2. Description of the Related Art
A boost pressure control system utilizing an air bypass valve is commonly used for an engine supercharged by a mechanical supercharger (i.e., a supercharger driven by a crankshaft of the engine).
This type of pressure control system usually comprises an air bypass passage connecting the inlet and discharge sides of the supercharger, and an air bypass valve installed in the air bypass passage.
The boost pressure of the engine is controlled by adjusting the degree of opening of the air bypass valve in accordance with an engine load.
Namely, when the engine load is lower, the degree of opening of the air bypass valve is increased so that the amount of air recirculated from the discharge side of the supercharger to the inlet side thereof through the air bypass valve is increased, and by increasing the recirculated air flow, the pressure difference between the inlet and discharge sides of the supercharger, and thus the boost pressure, is reduced. Conversely, when the engine load is higher, the degree of opening of the air bypass valve is decreased to increase the boost pressure.
This type of boost pressure control system is disclosed, for example, by Japanese Unexamined Patent Publication No. 62-276220.
In this control system, the degree of the openings of a throttle valve disposed downstream of the supercharger, as well as an air bypass valve, are adjusted according to the depression of the accelerator pedal (i.e., the load of the engine). Namely, when the engine is operated at a low load in which the supercharger is not operated, the air bypass valve is locked in the fully open position and the inlet air flow to the engine is controlled by the throttle valve alone. Conversely, when the engine is operated at a high load in which the supercharger is operated, the throttle valve is kept fully open and a boost pressure and inlet air flow are controlled by the air bypass valve alone. Further, the degree of opening of the air bypass valve is changed according to a speed of the engine, to ensure that the changeover of the inlet air control from the throttle valve to the air bypass valve is smooth.
A boost pressure control system of a similar type is also disclosed in Japanese Examined Utility Model Publication No. 61-14591. In this system, the degree of opening of the air bypass valve is adjusted in accordance with a pressure in an inlet air manifold of the engine (i.e., a load of the engine), such that the degree of opening of the air bypass valve is increased as the engine load is reduced.
When the load of the engine is reduced, the air bypass valve is fully opened at the same time as the supercharger is stopped.
As disclosed in the prior art, the mechanical supercharger is usually driven by the engine crankshaft via a magnetic clutch, whereby the supercharger is operated or not operated during the operation of the engine.
For example, when the engine is operated at a high load in which a supercharging is required, the magnetic clutch is made "ON" (i.e., connected) to operate the supercharger, and when the engine load becomes lower than a predetermined value, the magnetic clutch is made "OFF" (i.e., disconnected) to stop the operation of the supercharger. Namely, disconnecting the supercharger when a supercharging is not required reduces a power loss incurred when the supercharger is being driven, and therefore, the fuel consumption by the engine is improved.
In the boost pressure control system having a mechanical supercharger with a magnetic clutch, preferably the speeds of the engine and the supercharger coincide as much as possible when the magnetic clutch is made ON.
This is because, if there is a large difference between these speeds, a large starting torque caused by an inertial mass of the charger rotors is exerted on the engine crankshaft at the moment the magnetic clutch is made ON, and this is a cause of an undesirable engine output torque shock.
In the boost pressure control system disclosed by the above 62-276220 Publication, the air bypass valve is sometimes kept fully open when the supercharger is not operated. In this condition, the rotation of the rotors of the supercharger is completely stopped, because all of the inlet air flows through the air bypass passage.
Consequently, when the magnetic clutch is made ON, a large difference exists between the engine speed and the supercharger rotor speed, and thus an unwanted torque shock occurs.
In the system disclosed by the 61-14591 Publication, it is possible to prevent this torque shock by controlling the air bypass valve such that the air bypass valve is not fully open when the magnetic clutch is made OFF. This can be accomplished by setting the air bypass valve to a not fully open state until the load becomes much lower than the load at which the magnetic clutch is made ON or OFF.
By keeping the air bypass valve partially closed, the flow resistance in the air bypass passage is kept higher and a part of the inlet air flows through the supercharger. This air flow rotates the rotors of the supercharger, and therefore, the difference in the speeds of the engine and the rotors is kept low while the magnetic clutch is OFF, and thus the resultant torque shock when the clutch is made ON is reduced.
However, a problem arises if the air bypass valve is partially closed when the clutch is OFF in a system using a magnetic clutch, in that usually it is necessary to incorporate a time delay in the switching OFF action of the magnetic clutch. This time delay is necessary to avoid a frequent ON and OFF operation of clutch due to temporary changes of the engine load, because such a frequent ON and OFF operation of the clutch will lead to excessive wear and shorten the service life of the clutch.
By incorporating the time delay, the supercharger continues to operate for a predetermined time after the engine load is within a region in which a supercharging operation is not required. If the air bypass valve is not fully opened while the engine load is within this region, the supercharging operation is carried out while the air bypass valve is partially closed. This causes an unnecessary increase in the boost pressure in this region, and thus the fuel consumption of the engine is worsened due to an unnecessary compression operation by the supercharger.
To solve these problems, it is necessary to control the air bypass valve such that:
(1) When the engine is operated in the region in which supercharging is not required (hereinafter called "non-boost operation region"), the air bypass valve is partially closed after the magnetic clutch is made OFF, to maintain a rotation of the rotors of the supercharger by the air flow; and
(2) When the engine load is in the non-boost region, the air bypass valve is fully opened to reduce the amount of compression required from the supercharger if the supercharger is operating.
Such a control, however, cannot be accomplished by the boost pressure control systems of the prior arts.